.slidebar {
    width: 100%;
    height: 100%;
    position: relative;
}

.slidebar > .mask {
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    z-index: 1;
    background-color: black;
    opacity: 0;
}

.slidebar > .con {
    position: absolute;
    width: 75%;
    height: 100%;
    top: 0;
    z-index: 2;
    background-color: blue;
}